Sense amplifiers are commonly employed to sense the states of circuits such as memory cells and to amplify currents or voltages representative of such states. FIG. 1 shows a conventional sense amplifier 100 connected to a column of memory cells 1,2, . . . M. Sense amplifier 100 has input nodes Vin and Vin coupled respectively to emitters of bipolar transistors Q1 and Q2 and to current sources I1 and I2. The bases of transistors Q1 and Q2 are coupled to node V3, which is connected through a current source I3 to ground and connected through a resistor R0 to a supply voltage Vcc. The collectors of transistors Q1 and Q2 are connected to output nodes Vout and Vout of sense amplifier 100.
Bit lines BL and BL are coupled to input nodes Vin and Vin so that sense amplifier 100 can sense currents on bit lines BL and BL during reading of one of the memory cells 1,2, . . . M. To read a selected one of memory cells 1,2, . . . M, a voltage is asserted on a selected one of word lines WL1 to WLM, which causes one of the pairs of pass transistors PT1, PT1' through PTM, PTM' to connect the selected memory cell to bit lines BL and BL. For example, when word line WLM is asserted, pass transistors PTM and PTM' connect memory cell M to bit lines BL and BL, respectively. Depending on whether memory cell M is in a first or a second state (stores a "1" or a "0"), bit line BL will be driven high or low and bit line BL will be driven low or high, respectively when pass transistors PTM and PTM' are enabled. The driving of the bit lines BL and BL by a memory cell causes currents on bit lines BL and BL.
The currents pull the voltage on one of the nodes Vin or VIN higher pull the voltage on the other node Vin or Vin lower, and eventually change voltages on nodes V1 and V2, currents in transistors Q1 and Q2, and voltages on output nodes Vout and Vout. The rate of the change in voltage on output node Vout or Vout is related to the rate at which transistor Q1 or Q2 change its base-to-emitter voltage. The rates of change in the emitter voltages on nodes V1 and V2 are limited by the parasitic capacitances C1 and C2 which are charged/discharged as emitter voltages change. The change in base voltage of both transistors Q1 and Q2 is relatively insignificant because node V3 is coupled to Vcc via a resistor R0, and the current through resistor R0 is nearly constant. (Current source I3 typically provides a constant current which is much larger than the changes in base currents of transistors Q1 and Q2.)
One way to increase the rates of change in output voltages on nodes Vout and Vout is to increase the currents supplied by current sources I1 and I2 and thereby increase the amplification of transistors Q1 and Q2. Although increasing current increases the rate of change in the output voltages, increasing current also undesirably increases the power consumption of sense amplifier 100. Accordingly, there is a need for a current sense amplifier which more quickly provides desired output voltages without a corresponding increase in power consumption.